1. Field of the Invention
The present invention relates to an optical fiber for preserving the plane of polarization. More specifically, the present invention relates to a stress imparting type optical fiber for preserving the plane of polarization which may be used for optical communication or optical sensors and has an excellent plane of polarization preserving property even under wet heat conditions.
2. Background Art
There are various types of optical fibers for preserving plane of polarization (hereinafter also referred to as polarization plane preserving optical fibers) in which the stress distribution in a single mode optical fiber is anisotropic, and particularly, one having a stress imparting part in its cladding has been widely used. The stress imparting type polarization plane preserving optical fibers may be categorized as the PANDA type, the bow tie type, the oval jacket type, etc., based on their shapes. Among these the PANDA type polarization plane preserving optical fiber is particularly widely used because of its large double refractive index and excellent polarization plane preservability.
FIG. 1 is a diagram showing an example of a conventional PANDA type polarization plane preserving optical fiber. In FIG. 1, the PANDA type polarization plane preserving optical fiber 4 includes a core 1 having a large refractive index, a cladding 2 which is formed around the core 1 so as to be concentric with the core, and stress imparting parts 3 which are disposed in the cladding 2. The stress imparting parts 3 are placed symmetrically with respect to each other with respect to the core 1 in the cladding 2. Each of the stress imparting parts 3 has a circular cross-section and its refractive index is lower than that of the cladding 2.
The stress imparting part 3 is made by using a material having a thermal expansion coefficient larger than that of a material used for forming the cladding 2. In particular, a B2O3xe2x80x94SiO2 glass is widely used for the stress imparting part 3. Since the thermal expansion coefficient of B2O3 is larger than that of SiO2, the stress applied to the core 1 increases as the proportion or the concentration of B2O3 increases, and accordingly, the double refractive index thereof is increased.
Japanese Unexamined Patent Application, First Publication (Kokai), No. Hei 8-101323, for example, discloses a polarization plane preserving optical fiber having a stress imparting part whose B2O3 concentration increases toward its center. Accordingly, the distribution of B2O3 in the stress imparting part of the polarization plane preserving optical fiber spreads out from its center toward the outer periphery portion thereof. In the above publication, it is also disclosed that the B2O3 concentration at the peripheral portion of the stress imparting part is preferably 15 wt. % or less so that the thermal expansion coefficient of the stress imparting part 3 becomes closer to that of the cladding 2. In this manner, the concentration of stress at the interface between the two may be eliminated and the generation of cracks may be prevented.
However, if the concentration of B2O3 at the center of the stress imparting part is high as in the polarization plane preserving optical fiber disclosed in the Japanese Unexamined Patent Application, First Publication, No. Hei 8-101323, the-stress imparting part may be dissolved under wet heat environmental conditions since B2O3 is deliquescent. Accordingly, a concave portion may be generated in the stress imparting part. If a concave portion is generated in the stress imparting part, the polarization plane preserving property of the optical fiber is deteriorated and a polarization crosstalk may be induced.
An object of the present invention is to provide a polarization plane preserving optical fiber having an excellent polarization plane preserving property in which no concave portion is generated in its stress imparting part even under wet heat conditions.
The above object may be achieved by an optical fiber for preserving plane of polarization including a cladding and a stress imparting part disposed in the cladding, the stress imparting part comprising a base material including B2O3. The average concentration of B2O3 in the base material of the stress imparting part is in a range between about 17 and 21 wt. %, and the maximum concentration of B2O3 in the base material is in a range between about 17 and 22 wt. %.
According to the above optical fiber for preserving plane of polarization, since the average concentration of B2O3 in the base material of the stress imparting part is in a range between about 17 and 21 wt. %, and the maximum concentration of B2O3 in the base material is in a range between about 17 and 22 wt. %, the stress imparting part is not degraded even under wet heat conditions. Accordingly, the optical fiber has excellent polarization plane preserving properties, and hence, its polarization crosstalk value is not lowered. Also, cracks are not generated in the optical fiber due to the difference in the thermal expansion coefficient of the interface between the stress imparting part and the cladding.